Enterotoxigenic Bacteroides fragilis (ETBF) strains, which produce a 20 kDa zinc metalloprotease toxin (BFT), have been associated with diarrheal disease of animals and young children. Studying a collection of ETBF and non-toxigenic B. fragilis (NTBF) strains, we found that: 1) The bft gene and a second metalloprotease gene (mph) are contained in a ca. 6-kb pathogenicity island (termed B. fragilis pathogenicity island or BfPAI). 2) The BfPAI is integrated between genes, which encode proteins sharing significant homology to mobilization proteins encoded by Bacteroides spp. mobilizable plasmids. 3) The G+C content of the mobilization genes flanking the BfPAI (49 percent) differ substantially from those of the BfPAI (35 percent) and the rest of the B. fragilis DNA (42 percent), indicating that the BfPAI is contained in an additional foreign genetic element, 4) There are three major populations of B. fragilis strains: pattern I strains, containing the BfPAI and flanking element, all are ETBF strains; pattern H strains, lacking the BfPAI and flanking element, all are NTBF strains; and pattern III strains, containing the flanking element but lack the BfPAI, all are NTBF strains, and 5) The BfPAI and its flanking regions are necessary to bft expression. Based on these results, we hypothesize that the BfPAI and flanking regions are important to the pathogenesis of ETBF strains, and that ETBF strains evolved by acquisition of the flanking element and the BfPAI. The long-range goal of this project is to investigate the molecular evolution of ETBF strains and to further understand ETBF disease pathogenesis. The specific aims of this proposal are: 1) To characterize the BfPAI and its flanking region; and 2) To investigate the molecular evolution of ETBF strains. Our results will advance our understanding of the molecular pathogenesis of ETBF infections and will serve to delineate how ETBF originated during evolution.